Solder material for semiconductor device

ABSTRACT

A lead-free solder has a heat resistance temperature which is high and a thermal conductive property which is not changed in a high temperature range. A semiconductor device includes a solder material containing more than 5.0% by mass and 10.0% by mass or less of Sb and 2.0 to 4.0% by mass of Ag, and the remainder consisting of Sn and inevitable impurities. A bonding layer including the solder material, is formed between a semiconductor element and a substrate electrode or a lead frame.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. application Ser.No. 15/688,404 filed on Aug. 28, 2017 which is a Continuation of PCTApplication No. PCT/JP2016/073406 filed on Aug. 9, 2016, and claimsbenefit of foreign priority to Japanese Patent Application No.2015-184264, filed on Sep. 17, 2015 and Japanese Patent Application No.2016-007022, filed on Jan. 18, 2016, the entire contents of which areincorporated by reference herein.

BACKGROUND OF THE INVENTION Technical Field

The present invention relates to a solder material, and moreparticularly, relates to a highly reliable solder material used forbonding of a semiconductor device.

Background Art

In recent years, Pb-free solders containing no lead components havereplaced Sn—Pb-based solders from an environmental viewpoint. Amongavailable lead-free solders of different compositions which are usableas a solder material applied to an IGBT module (power module) or othersuch semiconductor devices, a Sn—Ag-based Pb-free solder is oftenadopted because of relatively well-balanced properties particularly interms of bondability (solder wettability), mechanical characteristics,thermal resistance, etc. as well as due to the fact that it has beenactually applied to products.

It is known that in a semiconductor device having a layered bondingstructure containing a radiator plate, an insulating substrate placed onthe sink, and a semiconductor element attached thereonto by solderbonding, a solder bonding structure is used, in which a Sn—Sb-basedsolder as a high-temperature lead-free solder is applied to a lowerbonding portion, and a lead-free solder is applied to an upper bondingportion, the lead-free solder having such a composition that an elementsuch as Cu is added to a Sn—Ag-based solder having a lower melting pointthan the Sn—Sb-based solder (see, for example, Patent Literature 1).

Also, the following structure is known as well: a lead frame, alsoserving as a heat spreader, is bonded as a wire member onto an upperelectrode of a semiconductor element (IGBT) mounted by soldering onto aninsulating substrate, and heat generated in a semiconductor element islet to dissipate to the lead frame so as to prevent the generated heatfrom accumulating in a certain portion (see, for example, PatentLiterature 2).

A Sn—Sb—Ag-based tape- or wire-like solder material, having high rollingproperty at 170° C. and high cold forming property, has been also knownas a solder material effective to avoid cracking at high temperatureresulting from heat generation of the semiconductor element (see, forexample, Patent Literature 3).

LIST OF PRIOR ART REEFRENCES Patent Literature

Patent Literature 1: JP 2001-35978 A

Patent Literature 2: JP 2005-116702 A

Patent Literature 3: JP H7-284983 A

SUMMARY OF INVENTION Technical Problem

MOS or IGBT elements called a power semiconductor generate heat bythemselves during operations, and reach high temperatures. Throughrepetitive heat generation and cooling, the solder-bonded elementrepeatedly suffers from distortion at a soldered portion andconsequently deteriorates. It is preferable to use a solder alloyexcellent in heat radiation for bonding a semiconductor element thatoperates at high temperatures. A SnAg-based solder material as a typicalPb-free solder increases heat resistance and lowers heat radiationcharacteristics as the temperature rises. In case of using a SnAg-basedsolder material that decreases thermal conductivity rate at hightemperature at a bonding portion of a power semiconductor subject toheat cycles for a long time, if a larger amount of power is applied, thesemiconductor may generate more heat.

In recent years, demand for power semiconductors with high currentspecifications has increased and accordingly, elements tend to generatea large amount of heat. Also, there is increasing demand for in-vehiclepower semiconductors or other such devices capable of operating at anenvironmental temperature of over 175° C. In such circumstances, lowthermal conductivity rate of a solder could be a bottleneck to powersupply relative to an applicable output of the element. In case thethermal conductivity rate of a solder lowers when the elementtemperature rises from the room temperature to high temperature due toself-heating or environmental temperature, a chip cannot easily releasethe heat. Consequently, the chip temperature further increases.Currently, it is earnestly desired to ensure that the maximum of powerapplied to an element is used so that the element can be used even ifgenerating heat with the temperature closer to a melting point of asolder. In order to meet such a demand, a solder material that is lesslikely to lower its thermal conductivity rate at high temperature isrequired.

Solution to Problem

The inventors of the present invention have made extensive studies andfound that if Sb is further added to the SnAg-based solder and theresultant is controlled within the range of specific compositionpercentages, a preferable solder material is achieved, which has no fearof lowing thermal conductivity rate along with the temperature rise andexcels in bonding characteristics such as wettability. In this way, theinventors have accomplished the present invention.

More specifically, according to one aspect of the present invention, asolder material comprises more than 5.0% by mass and 10.0% by mass orless of Sb, 2.0 to 4.0% by mass of Ag, and the remainder consisting ofSn and inevitable impurities.

It is preferred that the solder material comprising Sb, Ag and Snfurther comprises more than 0 and 1.0% by mass or less of Ni.

It is preferred that the solder material comprising Sb, Ag and Snfurther comprises 0.1 to 0.4% by mass of Ni.

It is preferred that the solder material comprising Sb, Ag and Snfurther comprises more than 0 and 1.0% by mass or less of Si.

It is preferred that the solder material comprising Sb, Ag and Snfurther comprises more than 0 and 0.1% by mass or less of V.

It is preferred that the solder material comprising Sb, Ag and Snfurther comprises more than 0 and 1.2% by mass or less of Cu.

It is preferred that any one of the solder materials further comprises0.001 to 0.1% by mass of P.

It is preferred that any one of the solder materials further comprises0.001 to 0.1% by mass of Ge.

In any one of the above solder materials, it is preferred that a thermalconductivity rate at 100° C. to 200° C. is not lower than a thermalconductivity rate at 25° C.

According to another aspect of the present invention, a semiconductordevice comprises a bonding layer in which any one of the above soldermaterials is melted, between a semiconductor element and a substrateelectrode or a lead frame.

It is preferred that in the semiconductor device, the semiconductorelement is a SiC semiconductor element.

Advantageous Effects of Invention

The solder material according to the present invention preferablyincreases thermal conductivity rate along with the temperature risewithout decreasing it along with the temperature rise. Thus, it has highheat radiation characteristics and provides an effect of increasing aheat fatigue life. Also, the solder material according to the presentinvention has a high wettability, and can suppress the generation ofvoids in the solder bonding layer to a lower level. The solder materialaccording to the present invention is particularly preferable for use ina solder die bonding portion used at the temperature condition, Tr=0.6or more, which corresponds to the melting point of the material. Notethat Tr represents a ratio of the operation temperature to the meltingpoint and is expressed by Tr=Tm/Tj where Tm indicates the melting pointand Tj indicates the operation temperature (the unit is K for both).Here, the void means a gap formed inside the solder bonding layer or atthe bonding interface. If the wettability between the solder and thebonding member is low at the bonding temperature, there is a problemthat voids easily occur in such a way that the air or other gases aretaken in or the solder is solidified while being recessed. The presentinvention is advantageous in that voids are less likely to occur.Moreover, the solder material according to the present invention furthercontains a predetermined amount of Ge to prevent oxidization of Sn andimprove the wettability.

Also, the semiconductor device including the solder material accordingto the present invention as a bonding layer has high heat radiationcharacteristics and thus is suitable for the application where anelement that generates a large amount of heat is mounted or the use athigh environmental temperatures. In addition, the solder materialensures reduction in size and costs of the device. Also, few voids areformed in the bonding layer, whereby the service life is increased.Hence, the solder material is preferably applicable to electronicdevices with large current specifications, for which demands areincreasing, especially for a wide variety of semiconductor devices, fordie bonding of the semiconductor device, bonding between terminals, andbonding between the other members.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a conceptual diagram showing an example of a semiconductordevice in which a solder material according to the present invention isused as a bonding layer.

FIG. 2 is a graph showing a relationship between each temperature andthermal conductivity rate in a solder material according to the presentinvention and that of Comparative Example where the thermal conductivityrate at each temperature is a normalized value based on the thermalconductivity rate at 25° C.

FIG. 3 is a graph showing a relationship between a normalized value of afracture life and the probability of failure in a solder materialaccording to the present invention and that of Comparative Example.

FIG. 4 is a photograph showing a wettability test result of a soldermaterial according to the present invention.

FIGS. 5A, 5B, and 5C are photographs showing a thermal shock test of asolder material according to the present invention.

DESCRIPTION OF EMBODIMENTS

Embodiments of the present invention will be described below withreference to attached drawings. However, the present invention is notlimited by the embodiments described below.

First Embodiment: Sn—Sb—Ag Ternary Alloy

According to the first embodiment of the present invention, a soldermaterial contains more than 5.0% by mass and 10.0% by mass or less of Sband 2.0 to 4.0% by mass of Ag, and the remainder consists of Sn andinevitable impurities. The inevitable impurities generally imply Cu, Ni,Zn, Fe, Al, As, Cd, Au, In, P, Pb, etc. The solder material of thepresent invention is a lead-free solder alloy that does not contain Pb.As the solder material mainly consisting of Sn contains Ag, and Sb inthe above composition ranges, the wettability of the solder material isensured, and these elements influence a thermal diffusion path of thealloy constituting the solder material to keep its thermal diffusivityrate low. As a result, even if the temperature rises, the thermalconductivity rate of the alloy is not decreased so much.

It is more preferred that the solder material contains 6.0 to 8.0% bymass of Sb and 3.0 to 4.0% by mass of Ag, and the remainder consists ofSn and inevitable impurities. These composition ranges make it possibleto increase the thermal conductivity rate of the alloy along with thetemperature rise in addition to the above advantages.

Second Embodiment: Sn—Sb—Ag—Ni Quaternary Alloy

According to the second embodiment of the present invention, a soldermaterial contains more than 5.0% by mass and 10.0% by mass or less ofSb, 2.0 to 4.0% by mass of Ag, and more than 0 and 1.0% by mass or lessof Ni, and the remainder consists of Sn and inevitable impurities.Further adding Ni in the aforementioned addition ranges to thecomposition of the first embodiment gives an advantage that a thermaldiffusion path of the alloy is influenced to increase its thermalconductivity rate as well as improve the wettability, whereby theresultant bonding layer has a low void fraction. Also, Ni has a highmelting point and thus can increase the strength at high temperature. Inparticular, the above addition ranges are set so as to control themelting point of the solder material within a possible range of solidsolution since the melting point of the solder material exceeds 300° C.when Ni is added beyond the above range.

More preferably, the solder material contains 6.0% by mass to 8.0% bymass of Sb, 3.0 to 4.0% by mass of Ag, and 0.01 to 0.5% by mass of Ni,and the remainder consists of Sn and inevitable impurities. Thesecomposition ranges give an advantage of further lowering the meltingpoint of the solder material to 260° C. or less in addition to the aboveadvantages.

Third Embodiment: Sn—Sb—Ag—Si Quaternary Alloy

According to the third embodiment of the present invention, a soldermaterial contains more than 5.0% by mass and 10.0% by mass or less ofSb, 2.0 to 4.0% by mass of Ag, and more than 0 and 1.0% by mass or lessof Si, and the remainder consists of Sn and inevitable impurities.Further adding Si to the composition of the first embodiment gives anadvantage that a thermal diffusion path of the alloy is influenced toincrease its thermal conductivity rate as well as improve thewettability, whereby the resultant bonding layer has a low voidfraction. Also, Si has a high melting point and thus can increase thestrength at high temperature. In particular, the above addition rangesare set to ensure solid solution of Si, which is originally hard toachieve. Also, if Si that is a material having a high melting point, isadded more than the above range, the melting point of the soldermaterial excessively increases to over 300° C. in some cases.

More preferably, the solder material contains 6.0% by mass to 8.0% bymass of Sb, 3.0 to 4.0% by mass of Ag, and 0.1 to 0.4% by mass of Si,and the remainder consists of Sn and inevitable impurities. Thesecomposition ranges give an advantage of controlling the melting point ofthe solder material to 260° C. or lower in addition to the aboveadvantages.

Fourth Embodiment: Sn—Sb—Ag—Ni—Si Quinary Alloy

According to the fourth embodiment of the present invention, a soldermaterial contains more than 5.0% by mass and 10.0% by mass or less ofSb, 2.0 to 4.0% by mass of Ag, more than 0 and 1.0% by mass or less ofNi, and more than 0 and 1.0% by mass or less of Si, and the remainderconsists of Sn and inevitable impurities. By using the quinary alloy inwhich Ni and Si coexist as additive elements, the following advantagecan be obtained: the interface strength and the strength of a bulkagainst high temperature increase, i.e., the strength to hightemperature increases due to the synergistic effect of Ni and Si.

More preferably, the solder material contains 6.0% by mass to 8.0% bymass of Sb, 3.0 to 4.0% by mass of Ag, 0.01 to 0.5% by mass of Ni, and0.1 to 0.4% by mass of Si, and the remainder consists of Sn andinevitable impurities.

Fifth Embodiment: Sn—Sb—Ag—V Quaternary Alloy

According to the fifth embodiment of the present invention, a soldermaterial contains more than 5.0% by mass and 10.0% by mass or less ofSb, 2.0 to 4.0% by mass of Ag, and more than 0 and 0.1% by mass or lessof V, and the remainder consists of Sn and inevitable impurities.Further adding V to the composition of the first embodiment gives anadvantage that a thermal diffusion path of the alloy is influenced toincrease its thermal conductivity rate as well as improve thewettability, whereby the resultant bonding layer has a low voidfraction. In particular, the above addition ranges are set because V hasa high melting point and thus can increase the strength at hightemperature. If V that is a material having a high melting point isadded more than the above ranges, the melting point of the soldermaterial excessively increases and sometimes exceeds, for example, 300°C. Also, solid solution might not be achieved easily.

More preferably, the solder material contains 6.0% by mass to 8.0% bymass of Sb, 3.0 to 4.0% by mass of Ag, and 0.01 to 0.08% by mass of V,and the remainder consists of Sn and inevitable impurities. Although themelting point of the solder material might excessively increase if Vthat is a material having a high melting point is added more than theabove ranges, it is possible by limiting to these composition ranges tocontrol the melting point to 250° C. or lower, in addition to the aboveadvantages. Also, adding an excessive amount of V produces an oxide,which is hardly mixed with metals, and voids are easily formed in somecases.

Sixth Embodiment: Sn—Sb—Ag—Cu Quaternary Alloy

According to the sixth embodiment of the present invention, a soldermaterial contains more than 5.0% by mass and 10.0% by mass or less ofSb, 2.0 to 4.0% by mass of Ag, and more than 0 and 1.2% by mass or lessof Cu, and the remainder consists of Sn and inevitable impurities.Further adding Cu to the composition of the first embodiment gives anadvantage that a thermal diffusion path of the alloy is influenced toincrease its thermal conductivity rate as well as improve thewettability, whereby the resultant bonding layer has a low voidfraction. The above addition ranges are advantageously set becauseparticularly in case of bonding a Cu member, a melting point is notincreased for the Cu member, that is, the melting point is insensitiveto its composition, and the composition margin is large with smallvariations in components. Also, these ranges are preferable in that Cuin a Cu plate is prevented from melting in the solder material.

More preferably, the solder material contains 6.0% by mass to 8.0% bymass of Sb, 3.0 to 4.0% by mass of Ag, and 0.1 to 0.9% by mass of Cu,and the remainder consists of Sn and inevitable impurities. Thesecomposition ranges give an advantage of particularly high wettability inaddition to the above advantages.

Seventh Embodiment: Sn—Sb—Ag—Ge Quaternary Alloy

According to the seventh embodiment of the present invention, a soldermaterial is an alloy containing more than 5.0% by mass and 10.0% by massor less of Sb, 2.0 to 4.0% by mass of Ag, and 0.001 to 0.1% by mass ofGe, and the remainder consists of Sn and inevitable impurities. Furtheradding Ge to the composition of the first embodiment has an advantagethat Sn is kept from oxidizing, the solder wettability largely increasesby such addition, and thus, the thermal diffusion path of the alloy isinfluenced. The addition amount of Ge is more preferably 0.003 to 0.05%by mass. By adding Ge in this range, GeO is not produced excessively butis produced in an appropriate amount so as to suppress the oxidizationof Sn the oxide of which cannot be easily reduced and removed. Also,this provides an effect of suppressing the void formation. The amount ismore preferably 0.003% by mass or more and less than 0.005% by mass.

More preferably, the solder material contains 6.0% by mass to 8.0% bymass of Sb and 3.0 to 4.0% by mass of Ag and contains Ge in any of theabove ranges, and the remainder consists of Sn and inevitableimpurities. These composition ranges make it possible to suppress theoxidization of Sn as well as increase the thermal conductivity rate ofan alloy along with the temperature rise.

Eighth Embodiment: Sn—Sb—Ag—Ge—Ni Quinary Alloy

According to the eighth embodiment of the present invention, a soldermaterial contains more than 5.0% by mass and 10.0% by mass or less ofSb, 2.0 to 4.0% by mass of Ag, 0.001% by mass to 0.1% by mass of Ge, andmore than 0 and 1.0% by mass or less of Ni, and the remainder consistsof Sn and inevitable impurities. The addition amount of Ni is morepreferably 0.1 to 0.4% by mass. Further adding Ni to the composition ofthe seventh embodiment gives an advantage that the interface strength ofthe solder can be improved together with the wettability improvingeffect of Ge. Another advantage is that Ni has a high melting point andthus can increase the strength at high temperature.

More preferably, the solder material contains 6.0% by mass to 8.0% bymass of Sb, 3.0 to 4.0% by mass of Ag, and 0.01 to 0.5% by mass of Ni,and the remainder consists of Sn and inevitable impurities. Thesecomposition ranges provide an effect of controlling the melting point ofthe solder material to 260° C. or less in addition to the aboveadvantages.

To give another modified example, P can be added to the solder materialof the first to eighth embodiments. For example, the material cancontain, for example, 0.001% by mass to 0.1% by mass of P. This aims atincreasing the wettability because P has an effect of suppressing theoxidization of the solder material. The solder material of the first tosixth embodiments can contain Ge in place of, or in addition to P. Thisis because Ge also has an effect of suppressing the oxidization of thesolder material and can influence the thermal diffusion path of thealloy. The addition amount of Ge can be set to 0.001 to 0.1% by mass inthis case, preferably, 0.003 to 0.02% by mass, and more preferably 0.003or more and less than 0.005% by mass. In case of adding Ge and Ptogether, their addition amounts can be appropriately chosen from theabove ranges. Both of Ge and P oxidize more easily than Sn, and if addedin these addition ranges, they can prevent oxidization of Sn and ensurethe wettability of the solder material.

In all of the first to eighth embodiments and the modified examplesthereof, the resultant solder material has such thermal conductiveproperty that the thermal conductivity rate detected at 100° C. to 200°C. is not lower than the one at 25° C. Here, “the thermal conductivityrate detected at 100° C. to 200° C. is not lower than the one at 25° C.”means that the thermal conductivity rate of the solder material detectedat a certain temperature from 100° C. to 200° C. is equal to or higherthan the one at 25° C. Insofar as the above condition is met at acertain temperature from 100° C. to 200° C., the thermal conductivityrate may gradually increase within the range of 100° C. to 200° C., orcan temporarily increase and then decrease in this range. Alternatively,it can remain unchanged. Owing to the above characteristics, the soldermaterial can be suitably used in a high temperature range. The soldermaterial of the present invention preferably has such a thermalconductivity rate as monotonously increases in the range of 100° C. to200° C. Moreover, it is preferred that the thermal conductivity rate ofthe solder material at a certain temperature from more than 25° C. to100° C. is also equal to or higher than the one at 25° C., but in therange of more than 25° C. to 100° C., the thermal conductivity rate maybe slightly decreased temporarily. For example, the following soldermaterial can be also given as a preferred example of the presentinvention: the solder member has such thermal conductive property that arelationship between the temperature and the thermal conductivity rateis expressed by a curve that is convex downward and preferably has atleast one inflection point in the range of more than 25° C. to 100° C.,and λ_(IP)/λ₂₅=0.9 or more where λ₂₅ indicates the thermal conductivityrate at 25° C. and λ_(IP) indicates the thermal conductivity rate at theinflection point. In any case, the measurements of thermal conductivityrate involve an error of about 10%. Note that the thermal conductivityrate λ can be determined by a steady state method such as a temperaturegradient method or an unsteady method that determines a thermaldiffusivity rate using a laser flash method, a hot wire method, etc.More specifically, the thermal conductivity rate λ can be derived from:λ=α×ρ×Cp  (Expression 1)where ρ indicates the density, Cp indicates specific heat, and αindicates a thermal diffusivity rate

The density can be calculated by the Archimedes' method, the specificheat can be calculated by a DSC (Differential scanning calorimetry)method, and the thermal diffusivity rate can be calculated by the laserflash method. Note that the thermal conductivity rate λ can be measuredby using a method compliant with JIS R1611, R1667, H7801, H8453, etc.

In all of the first to eighth embodiments and the modified examplesthereof, the solder material of the present invention can be prepared bymelting materials selected from Sn, Sb, Ag, and additive elements or amother alloy containing the materials in an electric furnace inaccordance with an ordinary method. It is preferable to use materialswith a purity of 99.99% by mass or higher.

According to the first to eighth embodiments and the modified examplesthereof, the solder material can be worked in the form of a plate-likepreform material or a solder paste prepared by powdering the materialsand mixing the powder with a flux. If the solder material is to beprovided in the form of a solder paste prepared by working the materialsinto the form of powder and mixing the powder with a flux, with respectto the particle size of the powdered solder, it is preferable to use apowdered solder of which the distribution of the particle size is in therange of 10 to 100 μm, and more preferably, 20 to 50 μm. For an averageparticle size, a powdered solder can be used of which the distributionof the particle size is in the range of 25 to 50 μm, for example, bymeasurement carried out by using a common laser diffraction andscattering-type particle-size-distribution measuring apparatus. For theflux, a freely selected flux can be used, and in particular, a rosinbased flux can be preferably used.

According to the first to eighth embodiments of the present inventionand the modified examples thereof, a member to be bonded by the soldermaterial can be a common electronic device member including a metalmember at least at a bonding surface. A typical example thereof is ametal member that functions as an electrode. An electrode memberconsisting of, for example, Cu, Ag, Au, Ni, or Fe, or an alloy thereof,can be used.

According to the first to eighth embodiments of the present inventionand the modified examples thereof, the solder material can be used for asemiconductor device, for example. In particular, it is usable for diebonding, bonding between terminals, bonding between a terminal and othermember, and for any other bonding. However, its application is notlimited to the above bonding. The member is preferably used particularlyfor die bonding for a device used in a high temperature environment,e.g., at 175° C. or higher. As the device used in a high temperatureenvironment, e.g., at 175° C. or higher, there are, for example, aninverter, a mega solar, a fuel cell, an elevator, a cooling device, anin-vehicle semiconductor device, etc. However, the present invention isnot limited thereto. The solder material is preferably used particularlyfor bonding a semiconductor element such as Si or SiC, or a Peltierelement in these devices. Note that the solder material of the presentinvention has the aforementioned thermal conductive property andpreferably, its thermal conductivity rate shows a roughly monotonousincrease at 100° C. or more and the material is melted almost at 240° C.or higher. Hence, the solder material is preferably applicable to adevice that may possibly be used under temperature conditions in such arange.

According to the first to eighth embodiments of the present inventionand the modified examples thereof, the solder material does not decreaseits thermal conductivity rate even at high temperature and has excellentheat radiation characteristics. Thus, even when used for die bonding ofan electronic device member having an element that will generate a largeamount of heat and/or an electronic device member used in a hightemperature environment, the solder material is less likely to causedistortions and ensures the formation of a highly reliable, long-lifebonding layer. Also, the solder material shows a high wettability andcan largely reduce a void fraction in the bonding layer. The voids mightlower the heat radiation characteristics, lead to a failure of thesolder due to locally generated heat, or make Si melted in case ofbonding a Si semiconductor element, but according to the presentinvention, such risks can be satisfactorily reduced.

Ninth Embodiment: Semiconductor Device

According to the ninth embodiment of the present invention, asemiconductor device includes a bonding layer between a semiconductorelement and a substrate electrode or a lead frame, the bonding layerbeing prepared by melting the solder material of the above first toeighth embodiments or the modified examples thereof.

FIG. 1 is a conceptual cross-sectional view of a power module as anexample of the semiconductor device of this embodiment. A power module100 has a laminate structure formed by mainly bonding a semiconductorelement 11 and a laminate substrate 12 onto a radiator plate 13 with abonding layer 10. The bonding layer 10 is formed by melting the soldermaterial of the above first to eighth embodiments or the modifiedexamples thereof under a predetermined bonding temperature profile andthen cooling the resultant. The radiator plate 13 is attached to a case16 incorporating an external terminal 15, and electrodes of thesemiconductor element 11 and the laminate substrate 12 are connectedwith the external terminal 15 via an aluminum wire 14. The module isfilled with a resin sealing material 17.

The semiconductor element 11 may be a Si semiconductor element or a SiCsemiconductor element, but the present invention is not limited thereto.For example, if the elements are mounted on an IGBT module, backelectrodes of the elements, which are to be bonded to a conductive metalplate of the laminate substrate 12, are generally made of Au. Thelaminate substrate 12 has a conductive metal plate made of copper oraluminum, which is formed on the front or back surface of a ceramicinsulating layer made of, for example, alumina, SiN, or the like. Theradiator plate 13 may be formed of metal having high thermalconductivity rate such as copper or aluminum. The solder material of thepresent invention is preferably used as a material for the bonding layer10 between the back electrode of the semiconductor element 11 and theconductive metal plate on the front side of the laminate substrate 12 oras a material for the bonding layer 10 between a conductive metal plateon the back side of the laminate substrate 12 and the radiator plate 13.The thickness, shape, etc. of the solder material used for forming thebonding layer 10 can be appropriately set according to its purpose orapplication without any particular limitation. However, the soldermaterial of the present invention has a higher wettability thanconventional techniques and is less likely to cause voids, and thus canbe thinned. As the thin solder material is low in heat resistance, it ispreferable to use the material in the semiconductor device. On the otherhand, if a chip of the semiconductor element 11 warps, the thickness ofthe solder material has to be increased accordingly. In this case, thereis a fear that voids are easily formed, but the high wettability makesit possible to avoid the formation of voids. In addition, the largethickness provides an effect of relaxing stress and thus ensures longlife. On this account, the solder material can be formed thin or thick,and the flexibility of design can be increased.

Note that the semiconductor device of this embodiment is illustrated asan example, and the semiconductor device of the present invention is notlimited to the device having the illustrated device structure. Forexample, in the semiconductor device structure with a lead frame asdisclosed in Patent Literature 2 by the applicant of the presentinvention, a solder material of the present invention can be used forbonding the lead frame and the semiconductor element. Alternatively, inthe semiconductor device having the structure as disclosed in JP2012-191010 A by the applicant of the present invention, the soldermaterial of the present invention can be used for bonding a copper blockand the semiconductor element. Furthermore, the solder material of thepresent invention can be also used for solder bonding (solder bondingportions) in the semiconductor device such as bonding between terminalsor bonding between the semiconductor element and the terminal, inaddition to the above die bonding.

Examples

(1) Measurements of Thermal Conductivity Rate and Wettability

A solder material of the present invention and a solder material ofComparative Example were prepared, and the thermal conductivity rate andwettability were measured for both of them. The thermal conductivityrate of the solder material was derived from Expression 1 above; thedensity was calculated by the Archimedes' method, the specific heat wascalculated by the DSC method, and the thermal diffusivity rate wascalculated by the laser flash method. Note that the measurements involvean error of about 10%. The thermal conductivity rate was measured at 25°C., 100° C., 150° C., 175° C., and 200° C. for samples of compositionsto determine if the thermal conductivity rate of the solder materialmeasured at 100° C., 150° C., 175° C., and 200° C. is equal to or higherthan the thermal conductivity rate at 25° C. The sample that satisfiesthis condition is given “Y (yes)”, or otherwise “N (no)”.

The samples for the wettability measurement were prepared by bonding a 9mm square Si chip and the conductive metal plate (copper) of thelaminate substrate 12 into a 110 μm-thick bonding layer, using thesolder material of the present invention and the solder material ofComparative Example. The bonding was carried out by keeping the materialfor two minutes at the liquidus temperature +30° C. as a melting pointof the solder material. The solder bonding portion was observed by SAT(Scanning Acoustic Tomography). From the SAT radioscopy image(transmission image), the void fraction was calculated on the assumptionthat the chip area is 100%. The solder material having the void fractionof 1.5% or less relative to the chip area is assumed to have awettability and given “Y (yes)”, and the material having the voidfraction of more than 1.5% is assumed not to have a wettability andgiven “N (no)”. The results are summarized in Table 1 below. Althoughdetailed data is not shown, the inventors of the present inventionmeasured the relationship between an Ag content and the surface tensionin a Sn—Ag-based solder by using a Wilhelm method. The measurementresult from the Wilhelm test reveals that the surface tension issmallest at around Sn-3.5 mass % Ag, particularly at Sn-3.0 mass % Ag toSn-4.0 mass % Ag. This result matches the measurement result of the voidfraction in this example. The above reveals that lowering the surfacetension is effective for reducing the void fraction. As for the solderwettability, it can be thought that if the surface tension is as smallas possible, the contact angle is decreased to ensure the highwettability with respect to the base member (member to be bonded).

TABLE 1 Characteristics Thermal Component mass % conductivity Sample No.Sn Sb Ag Cu other(s) liquidus rate Wettability 1 Sn—2Ag Bal. — 2 225.8 NN 2 Sn—3.5Ag Bal. —   3.5 220.5 N N 3 Sn—5Ag Bal. — 5 248.2 N N 4 Sn—7AgBal. — 7 281.5 N N 5 Sn—5Sb Bal. 5 — 243.5 N Y 6 Sn—13Sb Bal. 13  —284.6 Y N 7 Sn—13Sb—3Ag Bal. 13  3 281.2 Y N 8 Sn—7Sb—2Ag Bal. 7 2 246.6Y Y 9 Sn—6Sb—4Ag Bal. 6 4 230 Y Y 10 Sn—9Sb—3Ag Bal. 9 3 256 Y Y 11Sn—6Sb—4Ag—1.0Ni Bal. 6 4 Ni 1.0 237.5 Y Y 12 Sn—6Sb—4Ag—2.0Ni Bal. 6 4Ni 2.0 473.9 Y N 13 Sn—6Sb—4Ag—0.4Ni Bal. 6 4 Ni 0.4 237.5 Y Y 14Sn—6Sb—4Ag—0.01Ni Bal. 6 4 Ni 0.01 237.5 Y Y 15 Sn—6Sb—4Ag—2.0Cu Bal. 64 2.0 293 Y N 16 Sn—6Sb—4Ag—1.2Cu Bal. 6 4 1.2 256.3 Y Y 17Sn—6Sb—4Ag—0.1Cu Bal. 6 4 0.1 237.5 Y Y 18 Sn—6Sb—4Ag—0.001Ge Bal. 6 4Ge 0.001 230 Y Y 19 Sn—6Sb—4Ag—0.003Ge Bal. 6 4 Ge 0.003 230 Y Y 20Sn—6Sb—4Ag—0.03Ge Bal. 6 4 Ge 0.03 230 Y Y 21 Sn—6Sb—4Ag—0.001Ge—0.3NiBal. 6 4 Ni 0.3 235 Y Y Ge 0.001 22 Sn—6Sb—4Ag—0.003Ge—0.3Ni Bal. 6 4 Ni0.3 235 Y Y 23 Sn—6Sb—4Ag—0.03Ge—0.3Ni Bal. 6 4 Ni 0.3 235 Y Y Ge 0.0324 Sn—4Ag—0.5Cu—0.07Ni—0.01Ge Bal. 4 0.5 Ni 0.07 219 N Y Ge 0.01 25Sn—6Sb—4Ag—0.1Si Bal. 6 4 Si 0.1 230 Y Y 26 Sn—6Sb—4Ag—1.0Si Bal. 6 4 Si1.0 237 Y Y 27 Sn—6Sb—4Ag—0.3Ni—0.1Si Bal. 6 4 Ni 0.3 235 Y Y Si 0.1 28Sn—6Sb—4Ag—0.01V Bal. 6 4 V 0.01 230 Y Y 29 Sn—6Sb—4Ag—0.1V Bal. 6 4 V0.1 230 Y Y

In Table 1, Samples 1 to 7 and 24 correspond to Comparative Examples,and the others correspond to Examples.

Although a SnAg-based material is generally used as the solder material,the thermal conductivity rate of SnAg or SnAgCu-based alloy is decreasedalong with the temperature rise. Note that data about the thermalconductivity rate is not shown in Table 1. On the other hand, as for theSnSb-based material, the solid solution effect of Sb was observed inSn-5Sb, but an influence of βSn was large, and the thermal conductivityrate was slightly decreased along with the temperature rise, comparedwith an initial one. Here, detailed data is omitted. On the other hand,a Sn6Sb4Ag eutectic solder increased the thermal conductivity rate alongwith the temperature rise. As a result of adding 13% by mass of Sb toSn, the wettability lowered. This is supposedly because SbSn and Sb2Snprecipitated. Although the thermal conductivity rate is likely toincrease along with the temperature rise compared with an initial value,in case of adding 8% by mass or more of Sb to Sn, the thermalconductivity rate did not change regardless of an Ag content, etc.

FIG. 2 shows a relationship between the temperature and the thermalconductivity rate of a representative solder material of the presentinvention and a solder material of Comparative Example. The thermalconductivity rate is expressed as a normalized value based on thethermal conductivity rate at 25° C. It can be understood from thenormalized values in the graph that Samples 8, 9, 13, and 20 as thesolder material of the present invention increased the thermalconductivity rate from 25° C. to 200° C. compared with the one at 25° C.On the other hand, the solder material of Comparative Example obviouslylowered the thermal conductivity rate along with the temperature rise.

The following two mechanisms are conceivable as a thermal conductivemechanism:

-   1. Energy transmission by means of vibrations (phonons and lattice    vibrations) transmitted through crystal lattices; and-   2. Energy transmission by means of conduction electrons.

In general, the conduction electrons contribute to the thermalconductive in metals or alloys rather than the former. It is accordinglysupposed that when the temperature rises, electrons are scattered moreand more, leading to a decrease in electron conductivity and also inthermal conductivity rate. On the other hand, alloys constituting thesolder material of the present invention might possibly depend onlattice vibrations within a predetermined temperature range rather thanthe latter. More specifically, when the temperature rises, the latticevibrations increase and the electrons are scattered more and more.However, if the thermal conductive mainly depends on the latticevibrations, even in case the contribution of conduction electrons islowered, the thermal conductivity rate is thought to increase due to theenergy transmission by means of the lattice vibrations. Note that theabove explanation is merely given for understanding the presentinvention and the present invention is not limited to a specific theoryas above.

(2) Power Cycle Test

Power modules having the structure of FIG. 1 were prepared using aSnSbAg-based material as the ternary solder material of the presentinvention, and a SnAg-based material as the solder material ofComparative Example. These power modules were subject to a power cycletest under the condition that ΔTj=100° C., Tjmax=175° C., and one cycleconsists of 2 seconds for operation and 9 seconds for pause, so as toobtain the same heat generation temperature. FIG. 3 shows the resultsthereof. The fracture life is expressed by the number of cyclesnormalized based on Comparative Example. The vertical axis representsthe failure probability obtained by executing the power cycle test ontwenty modules, that is, a ratio of power modules failed due to anydamage of a solder. In FIG. 3, the numbers of cycles at a predeterminedfailure probability are plotted. It is determined whether any failureoccurs due to any damage of a solder, based on a change in heatresistance of the element. The rate of change in heat resistance of theelement was monitored and continuously checked. Then, the element thatgradually increases heat resistance was determined to involve damage ofa solder. Also, in order to exclude the failure of a wire bondingconnection for power supply to the element or the failure due to thedeterioration of the element, as for the element that increases the heatresistance and cannot be applied with a rated current, the soldercomposition was observed in cross-section, and the element having damageof a solder was counted as a failure.

According to the power cycle test or other such test methods which applya current and make the element generate heat to repeatedly turn on/offthe current supply based on the upper and lower limits of the heatgeneration temperature, a bonding layer warps due to heat generation.After the repetition of several hundred thousands of cycles, the soldermaterial deteriorates, leading to the fracture of the element. In thepower cycle test, the solder material initially has fine structure ofmetal compositions but as it deteriorates, compounds might precipitate,aggregate, and coarsen. On the other hand, the presence of the coarsecompounds means that more compositions having less displacement of βSnor compounds exist; these compositions exhibit similar behavior to puremetal and the thermal conductivity rate might be changed. This test alsoreveals that the solder material of the present invention increases theproduct life compared with the conventional solder material. This issupposedly because, although not intended to limit the present inventionby a specific theory, the SnSbAg-based material of the present inventionenables both of a solid solution effect of SnSb and a precipitationstrengthening effect of SnAg.

(3) Wettability Test

On a DCB (direct copper bonding) substrate, a plate solder having 9.5 mmsquare and 0.25 mm in thickness was placed and heated under an H₂reducing environment at 300° C. for 3 minutes to check the wettabilityof a solder. Note that the DCB substrate is a laminate substrateprepared by directly bonding a conductive metal plate made of copper orthe like on both sides of an insulating layer such as alumina-basedceramics using Direct Copper Bond method. The plate solder was preparedusing the compositions of Samples 9, 19, and 23 of Table 1. Two platesolders are placed on the DCB substrate per sample upon the test.

FIG. 4 shows the results thereof. FIG. 4 is a stereoscopic micrograph ofthe solder on the DCB substrate after heating. The solder materials ofthe present invention have the high wettability. Among these, it can bevisually observed that the oxidization of the solders of Samples 19 and23 can be suppressed and the wettability of the solder is improved,compared with the solder of Sample 9, by adding of Ge. It is alsoobserved that a wetting area of Sample 19 spreads in almost the sameshape as the one before melting, and the solder is almost white, thatis, Sn is not oxidized. Also, Sample 23 has the same or higherwettability. It is confirmed that the wettability is not at leastdecreased by the addition of Ni. Although not easy to find in thephotographs of Samples 19 and 23, the solder spreads all over the regionoutlined in square.

(4) Heat-Resistance Test

By using a 0.25 mm-thick plate solder having the compositions of Samples5, 9, and 13 of Table 1, the DCB substrate and the radiator plate werebonded under the same bonding condition as Item (3) above. A thermalshock test was executed to evaluate the heat resistance of a solder. Thetest condition is that the substrate was held for 10 minutes at −45° C.to 155° C., and subjected to 300 cycles each consisting of holding at−45° C. and holding at 155° C. for 10 minutes. Next, it was confirmed bya ultrasonic inspection microscope whether the solder bonding portion ispeeled off or cracked. FIGS. 5A, 5B, and 5C are photographs showing theresults. FIG. 5A is a photograph of the solder of Sample 5; FIG. 5B,Sample 9; and FIG. 5C, Sample 13. The black portions in the figuresindicate the solder bonding portions, and white portions indicatecracks, that is, failures of the solder. Regarding the determination asto cracks, in the solder material in which the white portions accountfor a large area in the initial solder bonding area indicated by thedotted frame, the cooling performance lowers and in turn, the solderbonding strength lowers. More specifically, the fewer the whiteportions, the higher the characteristics. Note that the white points atthe center, etc. of the micrograph for each sample represent voidspresent before the test and do not imply the sites where a failureoccurs in the solder upon the thermal shock test. The result of thethermal shock test reveals that heat resistance is higher in order ofSamples 13, 9, and 5 and the service life is increased by the additionof Ni.

If Ni is added to SnSbAg, it serves as a solidification nucleus for thesolidification of the SnSbAg composition, thereby providing finepolycrystal. Thus, it seems that the stress concentration due to thecold-heat cycle did not concentrate on the high-angle crystal grainboundaries with different crystal orientations, and the effect ofdispersing the stress was exerted by the polycrystal. Also, it isthought that the SbSnNi phase and CuNiSn phase crystallized atsolidification served to delay deterioration due to strengtheneddispersion to the SbSn phase and the coarsening of the main Sn phase andthe compound resulting from high temperature degradation. Note that theabove explanation is given merely for understanding the presentinvention, and the present invention is not limited to a specific theoryas above.

INDUSTRIAL APPLICABILITY

The solder material according to the present invention is used forgeneral electronic devices with high current specifications in bondingportions of a semiconductor chip and the like. Specifically, it issuitably used for bonding of packaged components such as integratedcircuits (ICs). In addition, it is suitably used for die bondingportions of components that generate a large amount of heat, e.g., powersemiconductor devices such as light-emitting diodes (LEDs) or powerdiodes, and further for die bonding portions for internal connections ofIC devices of general electronic components mounted on printed circuitboards.

REFERENCE SYMBOL LIST

-   10 bonding layer-   11 semiconductor element-   12 laminate substrate-   13 radiator plate-   14 aluminum wire-   15 external terminal-   16 case-   17 resin sealing material-   100 power module

What is claimed:
 1. A semiconductor device comprising a bonding layer inwhich a solder material is melted, between a semiconductor element and asubstrate electrode or a lead frame, wherein the solder materialcomprises: more than 5.0% by mass and equal to or less than 10.0% bymass of Sb; 3.0 to 4.0% by mass of Ag; 0.01 to 1.0% by mass of Ni; andthe remainder consisting of Sn and inevitable impurities, a thermalconductivity rate of the solder material at 100° C. to 200° C. being notlower than a thermal conductivity rate at 25° C.
 2. The semiconductordevice according to claim 1, wherein the solder material furthercomprises more than 0 and less than 1.0% by mass of Si.
 3. Thesemiconductor device according to claim 1, wherein the solder materialfurther comprises 0.001 to 0.1% by mass of Ge.
 4. The semiconductordevice according to claim 2, wherein the solder material furthercomprises 0.001 to 0.1% by mass of Ge.
 5. The semiconductor deviceaccording to claim 1, wherein the solder material further comprises0.001 to 0.1% by mass of P.
 6. The semiconductor device according toclaim 2, wherein the solder material further comprises 0.001 to 0.1% bymass of P.
 7. The semiconductor device according to claim 3, wherein thesolder material further comprises 0.001 to 0.1% by mass of P.
 8. Thesemiconductor device according to claim 4, wherein the solder materialfurther comprises 0.001 to 0.1% by mass of P.
 9. The semiconductordevice according to claim 1, wherein the content of Ni is 0.01 to 0.5%by mass.
 10. The semiconductor device according to claim 1, wherein thesemiconductor element is a SiC semiconductor element.